Predicting impact stiffness and rate of loading during human walking and heel-strike running

Heel-striking during human walking and running generates impacts beneath the foot. These impacts produce large forces over short time periods and occur millions of times per foot per year. In order to understand how the human body evolved to cope with these repetitive impacts, we must first understand how impact force parameters are generated beneath the foot in walking and running. Therefore, we used mass-spring models to predict stiffness and the rate of loading during the impact phase of gait. These models were tested on 20 human subjects walking and running on a rigid surface and four substrates of varying stiffnesses. All subjects walked and ran at Froude numbers of 0.28 and 1.2, respectively. Three-dimensional kinematic and kinetic data were collected using Qualysis motion capture software and an instrumented treadmill. Results indicate that impact stiffness on various substrates can be predicted accurately using knowledge of substrate stiffness and impact stiffness measured a rigid surface. Results also indicate that rate of loading scales predictably with impact stiffness. Importantly, declines in substrate stiffness of 94% produce only 17% declines in impact stiffness during walking compared to 55% declines in impact stiffness during running. This finding suggests that the human foot plays a greater role than substrate in governing impact forces during walking compared to heel-strike running. The performance differences in walking versus running provide a biomechanical context for interpreting morphological changes thought to be related to resisting impact forces, including variations in hominin calcaneal morphology.

Funding for this study was provided by Hintze Charitable Foundation and VibramUSA